There are a number of different devices which have been proposed for indicating the presence of oil in water. However, they do not permit an exact determination of the oil content and, further, they suffer from such disadvantages as complicated design, need of calibration for salt contents in the water, different water temperatures, etc. A special problem of indicating the amount of oil in water is that the type of oil and the quality of the oil that is in the water may vary quite considerably. For instance there are many, many different types of crude oils and many, many different types of refined hydrocarbons that are shipped in tankers and which may be present in a refinery. Accordingly, any instrument that is intended for monitoring oil in water should desirably be independent of the quality of the oil. Only then can the recorded oil content serve as evidence proving whether discharged amounts of oil have been within permitted limits or not.